Wireless broadband channels are necessary for transmitting multimedia data from a satellite network to moving vehicles, such as airplanes. For this purpose, antennas must be installed on the vehicles and require small dimensions to enable installation beneath a radome, while satisfying the requirements of the sending characteristics for directional wireless data communication with the satellite (such as in the Ku, Ka or X band), since interference with neighboring satellites must be reliably precluded.
When vehicles are moving, the relative position of the antenna with respect to the satellite changes continuously, so that constant tracking of the orientation of the antenna is required. The antenna is movable beneath the radome for this purpose to track the orientation at the satellite when the airplane is moving.
It is not only necessary to set the orientation of the antenna at the satellite, but also to compensate for polarization shifts of a received signal. The polarization shifts result from the changes in the vehicle's geographic position relative to the satellite and the angle of inclination of the vehicle-based antenna.
Bidirectional antenna systems for mobile satellite communication differ in the manner of the polarization thereof, among other things. For example, a distinction is made between linear and circular polarization, based on the satellite service. The polarization, in general, describes the orientation of the field lines in the plane orthogonal to the main lobe of the antenna. In the case of linear polarization, the field lines are always linearly oriented, and usually two orthogonal polarizations (horizontal and vertical) are used. In the case of circular polarization, the field lines follow a circular movement in the plane perpendicular to the main lobe. In this respect, a distinction is made between left hand (LHCP) and right hand (RHCP).
For satellite services using linear polarization, it is important that the planes of polarization of the antenna are correctly aligned with those of the satellite signal. Undesirable shifts in the planes of polarization result in polarization losses, signal interference, violation of regulatory requirements, and the like.
Shifts in the planes of polarization of the antenna and the satellite may be caused by a variety of effects such as limited mobility of the antenna positioning system, geographical location relative to the satellite, or movements of the vehicle. These may be corrected/compensated for by a polarization control unit (PCU), which pre-rotates the received signals, or the signals to be transmitted, corresponding to the present skew angle.
In many applications, 2-axes positioning systems are used. These systems can be used for the independent azimuth and elevation rotation of the antenna. The two axes of these systems form an orthogonal system and may allow the antenna to be aligned with any arbitrary point in the upper hemisphere of the three-dimensional space.
The 2-axes positioning systems, however, include a variety of drawbacks. For example, if the wireless communication system operates with electro-magnetic waves having a linear polarization, upon a rotation of the antenna the planes of polarization generally also rotate, so that the polarization plane of the target antenna no longer corresponds with the plane of polarization of the antenna located on the positioning system.
EP 2 425 490 B1 discloses a skew compensation controller for an individual polarization direction, which is based on a rotatable waveguide module. The user, however, often requires the horizontally and vertically polarized signals to be provided at the same time. Coupling the high-frequency signal out of the rotating waveguide module is critical since the guidance of the signal conductors represents a limitation in the outcoupling of two signal components.
DE 10 2014 113 813 discloses a dual-channel compensation of the polarization shift, in which a waveguide section can be rotated and switched back and forth between the signals so as to limit the necessary rotational angle for a full compensation. However, this requires several additional electronic components and cannot be used to process powerful transmission signals.
DE 20 2009 006 651 U1 discloses a micro-rotating joint having a rotatable round waveguide between two square waveguides, enabling the antenna to rotate.
U.S. Pat. No. 6,097,264 A discloses quad-ridge polarizers that are easy to produce.
G4UHP Circular and Rectangular Waveguide Septum Transformer Feeds discloses incoupling options into waveguides (See G4UHP Circular and Rectangular Waveguide Septum Transformer Feeds, 2014, URL: https://web.archive.org/web/20140326113551/http://g4hup.com/Personal/septum.html).